Make your most important decisions in the morning, before you experience "ego depletion."

"Freud speculated that the self, or ego, depended on mental activities involving the transfer of energy," the New York Times reports. "[His] experiments demonstrated that there is a finite store of mental energy for exerting self-control."

As the day wears on, your energy reserves are further depleted.

Your brain needs glucose in order to make good decisions.

"Even the wisest people won't make good choices when they're not rested and their glucose is low," Baumeister tells the Times. "That's why the truly wise don't restructure the company at 4 p.m. They don't make major commitments during the cocktail hour. And if a decision must be made late in the day, they know not to do it on an empty stomach."

Grocery retailers discovered this decades ago.

Researchers found that, "just when shoppers are depleted after all their decisions in the aisles — with their willpower reduced, they're more likely to yield to any kind of temptation, but they're especially vulnerable to candy and soda and anything else offering a quick hit of sugar."

Our finite supply of "decision-making power" means that making a series of decisions can be exhausting.

Which would explain why shopping is so tiring.

Researchers found that shoppers who "had already made the most decisions in the stores gave up the quickest" on a math test.